A boosted engine may offer greater fuel efficiency and lower emissions than a naturally aspirated engine of similar power. During transient conditions, however, the power, fuel efficiency, and emissions-control performance of a boosted engine may suffer. Such transient conditions may include rapidly increasing or decreasing operator torque demand, engine load, engine speed, or mass air flow. For example, when the engine output torque requested increases rapidly, a turbocharger compressor may require increased input (e.g., torque) from a turbocharger turbine, via exhaust mass flow, to deliver an increased boosted air flow to the engine. Such torque may not be available, however, if the turbine that drives the compressor is not fully spun up. As a result, an undesirable power lag may occur before the intake air flow builds to the required level.
To reduce the power lag, boosted engines may be operated in a blow-through mode wherein valve timing is adjusted to increase positive valve overlap. The increased valve overlap increases energy delivered to the turbine, reducing the overall time to boost. However, the inventors herein have recognized that when operating in the blow-through mode, which includes low engine speed and high engine load conditions, the engine may be prone to abnormal combustion events such as due to pre-ignition. The early abnormal combustion due to pre-ignition can cause very high in-cylinder pressures, and can result in combustion pressure waves similar to combustion knock, but with larger intensity. Such abnormal combustion events can cause rapid engine degradation.
While pre-ignition events may be mitigated by reducing the output of the boosting device or enriching the pre-ignition affected cylinder, as shown by Buslepp et al in US Patent Application 2013/0054109, these mitigating adjustments may not be viable options when operating in the blow through mode. As an example, during the blow through mode, the engine is typically operated with stoichiometric air-fuel ratio in the cylinder but lean at the exhaust catalyst due to the flow of air from the intake into the exhaust during the period of high valve overlap. During such conditions, pre-ignition mitigation using temporary cylinder enrichment may cause catalyst degradation due to overheating. Specifically, the excess oxygen available at the catalyst may react with the rich fuel injected following pre-ignition detection leading to an over-temperature condition at the catalyst. As another example, reducing the output of the boosting device may affect the engine torque output and turbo lag.
In view of these issues, the inventors have developed a method for addressing pre-ignition that may occur while the engine is operated with valve overlap. One example method comprises: while operating in a blow-through mode, reducing valve overlap in response to an indication of pre-ignition. In this way, pre-ignition occurrence during blow-through operation may be reduced.
In one example, an engine may be operated in a blow-through mode during conditions when turbo lag is likely, such as during a tip-in event. Therein, a variable cam timing device may be actuated to adjust a first intake or exhaust valve timing of one or more engine cylinders from a timing with no valve overlap to a timing with more positive valve overlap, such as with full positive intake to exhaust valve overlap. The flow of air from the intake to the exhaust via the cylinders is used to reduce time to torque. In response to an indication of pre-ignition received while operating in the blow-through mode, a controller may increment a first pre-ignition counter independent of a second pre-ignition counter that is incremented only in response to an indication of pre-ignition received while operating outside the blow-through mode. When the output of the first pre-ignition counter exceeds an upper threshold, load limiting actions may be taken. Specifically, engine load may be limited by reducing the blow-through of air, such as by adjusting the variable cam timing device to reduce the positive valve overlap. In one example, the engine may be temporarily operated with no valve overlap. In addition, the engine may be temporarily enriched. As the engine load is limited via reduction in valve overlap, the occurrence of pre-ignition events drops, and the first counter may be decremented. As the output of the first counter gradually drops, the valve overlap may be gradually increased until engine operation with higher (e.g., full) positive valve overlap is resumed.
As such, when the engine is not operating in the blow-through mode, the second, pre-ignition counter may be incremented and pre-ignition mitigating actions may be taken when the output of the second counter exceeds a threshold. The mitigating actions may include enrichment of the pre-ignition affected cylinder, as well as one or more additional cylinders. The actions may also include limiting of an engine load by reducing an intake throttle opening, or operating the waste gate at a more open position reducing overall boost. The second counter may then be decremented as the engine load is limited and the occurrence of pre-ignition events drops. As the second counter is decremented, the throttle opening (or wastegate opening) may be increased until stoichiometric engine operation with nominal throttle and wastegate settings are resumed.
In this way, an engine's propensity to pre-ignite while operating in a blow-through mode can be reduced. By enabling extra mass flow and enthalpy to be provided in the exhaust via the use of positive valve overlap, blow-through air can be advantageously used to expedite turbine spin-up and reduce turbo lag without degrading engine performance. By temporarily decreasing the amount of valve overlap in response to pre-ignition, catalyst overheating and engine degradation can be reduced. By adjusting valve timing to reduce the engine load in the blow-through mode, pre-ignition can be mitigated while enabling blow-through operations to be rapidly resumed. By extending the use of blow-through air, engine performance benefits are extended.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.